1. Field of the Invention
The invention relates to a method for controlling the output of electrical consumers connected to ac line voltage. The invention further relates to an arrangement for controlling the output of electrical consumers for carrying out this method.
2. Description of the Prior Art
Devices for controlling the output of electrical consumers which are connected to ac voltage, generally the line voltage, are known in a variety of forms and usually comprise a phase control circuit by which the firing angle of the electrical alternating quantity supplied to the consumer can be adjusted as desired, so that the supplied output can be regulated.
Phase control circuits of this kind are known, for example, from German reference DE 33 03 126 C2 which discloses a device for switch-on peak limiting in an engine control unit for the driving motor of a vacuum cleaner, which engine control unit is provided with a phase control circuit. Another example is disclosed in German reference DE 43 27 070 C1, which describes a device for regulating the power consumption of a vacuum cleaner. The ac voltage supplied to the electric motor driving the vacuum cleaner is regulated via a phase control circuit so that its value corresponds to the effective value of the motor voltage. The phase control circuits used for this purpose generally contain a triac which is connected to line voltage in series with the electrical consumer, in this case an electric motor, and supplies the electric motor with a discontinuous (sine) voltage depending upon the power desired.
Based on a circuit built from discrete components for the phase control, although the range of conceivable possibilities for realizing the circuit extends from optionally large-scale integration to a pure microprocessor control, the phase control in the trigger circuit for the triac comprises a resistor which is generally adjustable as a potentiometer or trimmer and a charging capacitor for firing the triac depending on the adjusted resistance. Thus, practically any desired intermediate outputs up to the full angle can be called up as desired by appropriate displacement of the firing angle.
However, a problem which always arises in phase control circuits of this kind, and which has recently come under increasing focus, is that even with comparatively small loads when these loads react ohmically, but also generally with increasingly higher possible maximum outputs of the electrical consumer (for the sake of clarity, the output of an electric heating device will be taken as an example hereinafter, although it is understood that the invention is applicable to any electrical consumer) there is a predetermined limit with respect to the harmonics generated overall by the consumer, including its control circuit. This limit can not be overcome in a simple manner.
In general, harmonics always occur in the absence of proportionality between the current and voltage. In the output control of an electric universal motor, harmonics are chiefly formed at first as odd-numbered harmonics which are derived in a first approximation from the quadratic dependency between current and voltage. But harmonics are also formed by, and in particular through, the phase control itself. These harmonics are moreover especially pronounced when the firing angle is approximately 90.degree., that is, when the permeability to current of the series triac is effected to some extent in the middle of the respective half wave as a result of corresponding firing.
Particularly when switching ohmic loads with triacs in ac line voltages, the short switching times of triacs which are commonly used today, result in very steep current increases which lead to considerable harmonic components as can be detected by Fourier analysis. The following numerical values can be taken as an exemplary point of departure, wherein it is understood that the invention is not limited by these values. Typical triac switching times of 5 .mu.sec and load currents between 10 and 15 amperes, for example, result in current increases of more than 2.5 amperes/.mu.sec. This causes high levels of interference precisely in the 100 to 300 kHz range, which levels already exceed permissible limits even with loads of only approximately 500 Watts.
Usually, such limits on the permissible harmonic content as mentioned above are determined by governmental regulations or will soon be incorporated in corresponding regulations. For example, the harmonic range is represented in Europe by the EMC or electromagnetic compatibility standard, as it is called, which to this extent also restricts the operation of ohmic loads in connection with phase controls.